In the technical realm, there are many areas where there is sometimes a need to allow only certain components of a signal to pass through, but to split other signal components from the signal. Such devices are generally referred to as filters.
For example, in the case of input radiation that has a wide energy spectrum, it is sometimes necessary to allow only a certain energy range to pass through the filter, but to split off other energy ranges from the radiation that is to be processed (to be “filtered”). Such a filter device for radiation is typically referred to as an energy filter. Sometimes, the term frequency filter is used, whereby the so-called de Broglie relation can be employed to convert the energy of radiation into a frequency and vice versa. This relates not only to photon radiation but also and especially to particle radiation (also called corpuscular radiation).
Especially in particle accelerator technology, there is regularly a need to allow certain energy ranges to pass through an energy filter, while other energy ranges have to be filtered out by the filter. This involves not only uncharged particles but also charged particles (for example, electrons, protons and heavy ions, or in very general terms, charged and/or uncharged leptons and/or hadrons). In the meantime, particle accelerator technology has developed beyond pure (basic) research and is now used routinely in a number of fields. Purely by way of example, mention should be made here of electron welding techniques, but especially of the medical use of particle radiation, for instance, in cancer treatment.
Particularly in cancer therapy, ions, specifically heavy ions (for instance, carbon ions, oxygen ions, neon ions, nitrogen ions and the like) have proven to be very advantageous since such heavy ions have a pronounced Bragg peak, thus making it not only possible to deposit a specific radiation dose in a way that is focused in the x-y-direction, but also to limit the dose deposition to a certain depth range (z-direction).
Up until now, such particle beams (that is to say, in particular, heavy ion particle beams) have been generated for use typically with linear accelerators, particle cyclotrons and/or particle synchrotrons. However, the requirements in terms of the equipment needed for such particle synchrotrons are quite extensive so that efforts are being made to cut back on these requirements. Moreover, particle beams that are generated by linear accelerators, cyclotrons and/or synchrotrons entail certain physical drawbacks. Furthermore, such accelerators are very large and not very energy-efficient in relation to the number of particles generated, which results in correspondingly high installation and operating costs.
A proposal for an alternative way to generate particle beams, in particular, heavy ion particle beams, consists of generating the particle beams using lasers. In this process, a high-energy laser is applied to a thin film. The actual acceleration procedure of the ions takes place directly behind the thin film, which is irradiated on the front with the laser light at an extremely high power density (typically in the range from 1021 Watt/cm2). The thermal energy thus deposited into the film brings about the acceleration of the ions due to thermal kinetic effects.
In particular, with this proposed accelerator concept—in contrast to the properties of particle synchrotrons or linear accelerators—ions occur that are released from an essentially punctiform initial position towards the outside in the shape of a bundle. Moreover, a broad spectrum of very different particle energies occur. Thus, it is desirable to focus the radiation bundle that is fanned open angularly and moreover, to filter out the useable energies. It would be especially preferable if the filtering could be variable, so that a depth modulation can be achieved in a simple manner when material is irradiated (for example, the tissue of a patient).
It has been found that, as a rule, existing concepts for the energy filtering of radiation from particle radiation entail considerable deficits, especially when they are used together with laser target film particle accelerators.